1. Field of the Invention
The invention concerns an x-ray tube of the type having at least one cathode to generate an electron beam, an anode at which the electron beam strikes and there forms a focal spot so that an x-ray beam emanates from the focal spot, an exit window through which the x-ray beam exits from the x-ray tube, and a backscatter electron trap in order to capture electrons backscattering from the anode. The invention also concerns an x-ray device with such an x-ray tube.
2. Description of the Prior Art
An x-ray tube of the above general type is described in United States Patent Application Publication No. 2008/0112538.
An x-ray tube normally has a cathode and an oppositely situated anode that are arranged in a vacuum housing. The cathode has a filament to emit electrons, and the electrons are accelerated in the direction toward the anode by application of a voltage between the anode and the cathode. The electrons strike a region of the anode that is designated as the focal spot, wherein their kinetic energy is transduced into heat and x-ray radiation (primary radiation). The x-ray radiation that is thereby generated exits from the vacuum housing through an exit window in the form of an x-ray beam (usable x-rays). An electron striking the anode experiences scattering processes at the atoms in the anode that both alter its direction of motion and emit energy. If the kinetic energy of the electron drops sufficiently, it is absorbed into the anode.
Such scattered electrons can also exit the anode again, such that a portion of the incident electrons exits the anode surface again. These electrons are designated as backscatter electrons. Some of the backscatter electrons may strike the anode again and some may strike additional components of the x-ray tube and there transduce their energy into radiation or heat. X-ray radiation generated by the backscatter electrons is designated as extrafocal x-ray radiation (extrafocal radiation) because it arises outside of the impact surface of the primary electron beam. A higher proportion of extrafocal radiation produces an increase in the blurriness of the optical focal spot and thus negatively affects the image quality.
Particularly in modern “unipolar” high power x-ray tubes for computed tomography, a backscatter electron trap (BSE trap) is necessary in order to capture the electrons backscattered from the anode. The trap has the primary purpose of capturing the energy stored in these backscatter electrons and thus keeping it away from the anode, since if this energy were absorbed in the anode it would be more difficult to cool. The BSE trap additionally offers the possibility of masking the unwanted extrafocal radiation with absorption filters located as close as possible to the location of generation of the used usable x-ray radiation, thus collimating the useful radiation. However, it is undesirably physically unavoidable that extrafocal radiation arises at the BSE trap upon impact of the electrons. The image quality can be degraded by the extrafocal radiation. Furthermore, the dose exposure of a patient is increased in an x-ray examination.